In printed circuit industry, photographic masks or stencils bearing a circuit pattern on a transparent substrate are known as phototools. Such a stencil, through which a photoresist can be selectively exposed by controlled irradiation as designed, provides an intricate complex image representing an electrical circuit. The image often consists of many fine lines and junctions spaced closely together as the advance of technology for higher resolution and smaller PCB size. During its use to make printed circuit boards, the phototool is placed face down on a photoresist layer and a contact print is made by exposing the photoresist to light, typically UV light, through the phototool, followed by separation of the phototool from the exposed photoresist layer. In this way, a single phototool can be repeatedly used to make multiple contact prints.
The continued use of the phototool can cause tiny scratches and abrasions on the phototool surface. The photoresists on which the phototool is placed are usually laminated on sheet copper (or other vapor-deposited copper substrates) and small burrs or rough edges of the copper sheet can cause scratches as the phototool is transferred from one photoresist to the next. The phototool is also frequently wiped with a soft cloth to make sure it is dust and lint free. Small particles of dirt can cause scratching as they are wiped across the phototool surface. Because of this general wear and tear on the phototool surfaced during normal use, the phototool must be frequently inspected to ensure line continuity. Depending upon the size and the intricacy of the phototool, such microscopic inspections can take 2 to 3 hours.
Ideally, the phototool must be cleanly removable from the exposed photoresist to prevent any transferring of photoresist to phototool and minimize contamination of the phototool. Various means of protecting the phototool have been described.
Due to the fact that phototools are vulnerable to scratching and that abrasion is a serious problem during the normal use of a phototool, protective films and overcoats having release performance are often employed to protect the phototool and allow repeated use of the phototool. For example, polysiloxane films coated with various kinds of pressure sensitive adhesives have been laminated to image-bearing surfaces of the phototool to protect the image and provide smooth release from photoresist. Because of their thickness required for the laminating process and minimized protection, however, the laminating films can cause optical distortion and only be used for PCB requiring limited resolution. In addition, the polysiloxane films are relatively soft and thus provide only limited scratch protection.
Thinner and harder protective coatings can be obtained by coating the surfaces of phototools with liquid compositions. Then, the thin liquid coating is hardened to yield the desired protective coat with improved scratch resistance. Epoxy silanes and acrylate esters (for example, polyurethane acrylates) have been used as protective hard coatings because of their resistance to abrasion. Many of these protective overcoats have limited release properties, however, and can therefore stick to the surface of the photoresist even when additional slipping agents are used, particularly when sticky photoresist materials such as high viscosity solder mask inks are present.
U.S. 2011/008733 and U.S. 2011/027702 (Qiu et al.) describe a hardcoat composition to be applied to the phototool with reduced surface energy for improved durable release from photoresist that comprises (a) one or more epoxy silane compounds, (b) one or more epoxy-functionalized perfluoropolyether acrylate oligomers, and (c) photoacid generator. Applicant's copending application U.S. Ser. No. 61/549,138, filed 19 Oct. 2011, describes hardcoat compositions comprising (a) an epoxy silane compound, (b) a reactive silicone additive, and (c) photo-acid generator for phototool protection and release performance.
Alternative approach for easier release of phototool from photoresist for repeated use is having low surface energy photoresist, which can be achieved by using low surface energy additive in photoresist.